Why this tool is useful
Use this calculator to verify IPv6 allocation boundaries and address capacity without rounding large counts or sending internal prefixes to a remote network service.
Developer & Data · runs locally
IPv6 Subnet Calculator
Calculate the first address, last address and exact address count for an IPv6 CIDR prefix using 128-bit browser arithmetic.
Ready. Your working data stays in this browser.
How it works
The validated address is converted to a 128-bit BigInt, host bits are cleared according to the prefix and the last address is derived by setting those host bits.
Practical example
For 2001:db8:abcd:12::1/64 the network is 2001:db8:abcd:12::/64 and contains exactly 18446744073709551616 addresses.
Validation
The calculation is deterministic and explains validation errors instead of silently changing invalid input.
Privacy boundary
Input and output remain in this tab. Copy and download happen through browser APIs without a server upload.
Result passport
Every run reports its tool mode, timestamp and input/output size so transformed data can be audited.